Advocating for your health is in no way being a pest. I second what @callalloo mentioned about, if this were for your child you wouldn’t hesitate to pester a doctor to get the best care. Well, your health is equally important.

So approach this in the perspective that the doctor works for you and is in a partnership with you to keep you healthy. Don’t apologize for asking questions. Knowledge is power. As a patient it’s natural to have questions about your health and the tests being given. In the case of this particular stress test, you see a cautionary red flag considering your symptoms. So be pleasant but remain firm in your resolve to get an answer. Have notes of what you want to say and why you have the concerns.

From experience, a reply for an email invitation to a patient portal is usually pretty quick. So you might want to check into that again as well. But definitely call. Do you ever get to speak with a receptionist or is it always voice mail?